sheesh... we admired your hair.... you're one of those guys that can pull it off.

Thanks, like I've said before, I get mixed reviews.

It's actually a pity I assumed that not everyone there was from this forum, at SITP I often get a "what forum?" instead of "I'm X", so I've grown weary of asking the question. We have atheists/skeptics from all over that attend who are not from this forum. As such we didn't even get in a chat. (I also assumed our guide/other guests may not be atheist and didn't broach that one either)

I suspected Faerie might be there, and might be the shortish glasses lady with kids, but Tweefo, I had no clue, and was a veritable astronomical information mine. So glad you came, as our guide didn't seem as prepared.

I'm in the throes of my next book (a sequel to my Moses book) and need some assistance from you astronomers: The world is destroyed and for 82 years a thick red dust cloud covers the whole planet; the descendants of the survivors of an Airbus 380 crash grow up in a world without the sun, moon and stars; these are legends passed down the generations. They do not know any longer how to count time except in a macro sense by seasons and neither do they have knowledge of longitude and latitude ; after year 82 the skies open up and all is revealed, and new gods are created etc. Now the rationalists among them start working on a system to measure the movement of the sun, moon etc. They have no libraries to refer to; no electricity, watches etc. They have in fact been cast back into a Bronze Age but minus celestial bodies.Question:If you are trekking (Exodus)south from the northern Hemisphere (Ethiopia) (thus Polaris is still visible but dips lower as you move south) toward the equator, how would you record every sunrise/sunset/movement of the stars at night to develop a calender and time system, taking into account winter/summer differences?

Being a sailor I have some knowledge of celestial navigation but this requires accurate time measurement and an almanac which these pilgrims do not have.Hope you can help!?

I'd like to point out that Addis Abbiba (the capitol of Ethiopia) is only 9o North of the equator. When the hero passes the equator then Polaris disappears below the horizon. 1 degree latitude is about 111km assuming a flat terrain, the classic spherical chicken assumption, at say a leisurely 15km per day then you're in the southern hemisphere in about 2 months or so.

Your civilisation will probably mark calendars the way ancient Egyptians did it, the day when a bright star rises at the same time as the Sun. In the Egyptian case it is the star Sirius, and for them it rose on June 21st. The day depends on your latitude, at Addis Abbiba it's around the 15th July according to Stellarium. This will be chosen as the first day of their calendar year. They will notice that it takes 365 days to rise again with the Sun. From this they can work out where Sirius will be in the evenings throughout the year and hence know what day it is by observing the position of Sirius on any given night.

After a while though they will notice that the seasons are not correlating with this calendar year so they'll start making adjustments by adding days to the calendar on some years to "re-sync".

thanx benguella: they crashed at roughly 10degrees 43'N and 37degrees 57" E I'm going to research Sirius and also how the ancient fleets of the Chinese admiral Zheng He in the 1400's were able to eventually establish longitude long before western navigators did. The Zhou Gong tower in China some 700 years old contains a thin vertical rod for observation of the stars on the local meridian combined with a water clock and laying stones between two parallel beds of water (to level)they measure the sun's noon shadow cast by a long metal pole. At equinox the sun would rise due east and set due west and at midday cast no shadow...they were actually able to calculate by measuring the length of the noon shadow, what day of the year it was!The adjustments you speak of need to account for the irregular motion of the earth round the sun etc.? In navigation before clocks, the oldest method to calculate was by observing lunar eclipses and elapsed time (Hipparchus, 190-120BC) but it is not known how local time was calculated then.

The adjustments you speak of need to account for the irregular motion of the earth round the sun etc.?

It takes Sirius 365.25 days to rise with the sun again. It's that annoying .25 that requires adding an extra day every 4 years. Your guys might not realise this at first so they may not readjust every 4 years, but maybe they'll add many days to a year many years later when Sirius is rising in Winter instead of Summer. Eventually they should figure out the 4 year cycle.

As for the Chinese tower that is very interesting. Do you know if the tower was built on the equator or where they sophisticated enough to build an "equatorial sundial" i.e. it was parallel to the Earth's equator? Note that the "no shadow at noon of an equinox" only happens on the equator or on an equatorial sundial properly aligned corresponding to it's latitude.

[edit]oh wait, you said Zhou Gong tower is in China so obviously not on the equator, so I assume it was aligned with the equator[/edit]

To be a complete PITA pedant (and mostly irrelevant to the point), it’s 365.2425 days (and very slowly falling). That’s why years that are exactly divisible by 100 (and therefore also exactly divisible by 4) are leap years only if they are also exactly divisible by 400. Thus, 1700, 1800 and 1900 weren’t leap years but 1600 and 2000 were.

As for measuring time, it’s likely that someone would have observed and thought about at least one form of regular motion such as a crude pendulum, the regular swaying of a tree in a steady wind, or the steady rate of a water wheel’s rotation. All that would be needed is a system of counting and numeration plus a minimum of mechanical ingenuity, and the rudiments of time measurement are in place. Based thereon, there’s scope for devising a novel time-tracking technique for people deprived of modern clocks…